Single molecule detection has become a reality in application domains such as healthcare. For instance, the use of techniques such as tethered particle motion (TPM) has made it possible to detect the binding of a single biomolecule, from hereon referred to as the analyte of interest, to a receptor anchored on the surface of a sensing device. The binding event typically alters the molecular dynamics of the receptor, which can be detected using optical or electrical detection techniques.
For instance, in case of the receptor comprising a nucleotide sequence such as a DNA fragment, prior to a binding event with a complementary nucleotide sequence the receptor may adopt a coiled-up or U-shaped (looped) conformation, e.g. due to the presence of a (partial) palindrome in the nucleotide sequence, with the conformation becoming more stretched or linear when a binding event takes place. This change in conformation affects, i.e. increases the volume of Brownian motion that the receptor inhabits. This change in volume may be detected by attaching a label to the receptor or the analyte of interest and recording the motion of the label.
Several examples of recording the trajectory of such a label, i.e. a metal nanoparticle or bead, are disclosed in a paper by H. R. C. Dietrich et al. in Journal of Nanophotonics, Vol. 3 (2009), pages 1-17. These detection techniques are based on an optical detection of the motion of the particle, including darkfield microscopy and the whispering gallery mode detection method, the latter method being particularly suitable for integration in semiconductor devices such as integrated circuits (ICs).
It is also possible to detect such changes electrically, as the aforementioned conformation change alters the effective dielectric constant directly above the surface of the sensing device. This effect may be utilized by defining the volume directly above the surface of the sensing device in which the receptor resides as the dielectric layer of a capacitor, such that the changes in dielectric constant are translated in a change in capacitance of the capacitor. In addition, when using metal beads as labels, the distance of the bead to the sensing surface is also correlated to the effective potential of the sensing surface, which, for instance when using the surface as the gate electrode of a transistor formed in the substrate of the sensing device, can be used to influence the conductivity of the transistor. Other suitable detection techniques are known to the skilled person.
It is particularly attractive to have a sensing device that is capable of implementing a high degree of multiplexing, such that the device can detect a large number of different analytes of interest in a single measurement procedure. An example of such a device is given in WO 2009/060379 A2, which utilizes the whispering gallery mode detection method for detecting a binding event. Particular attention is drawn to FIG. 6 and its detailed description in which the whispering gallery mode detection method is explained in detail. The sensing device disclosed in this patent application comprises a plurality of sensor active structures that are each functionalized with a different biomolecule such that the sensing device is capable of detecting a different analyte of interest at each sensor active region.
It is desirable to increase the degree of multiplexing to the highest possible extent, as a higher degree of multiplexing allows for more analytes of interest to be simultaneously detected. Currently, a sensing region is functionalized with its receptor, e.g. a biomolecule for binding to an analyte of interest by depositing the biomolecule onto the sensor region dissolved in a droplet of liquid. The dimensions of the droplet thus govern how many sensor regions may be defined onto a surface of a sensing device such an IC comprising a sensing surface. Currently, advanced deposition tools allow for the deposition of droplets having a 5 μm diameter at a 10 μm distance from each other. As the size of an IC ideally is kept as small as possible, e.g. for cost reasons, it will be appreciated that it is far from trivial to produce a sensing device in a cost-effective manner that allows for a high level of multiplexing.